Ok.... I'm going to jump on Stoo's band wagon a bit here..... Being in Europe it's hard to relate to the acceptance of obesity the way an American can.
I agree with him that you cannot be "fit" and "obese". These things seem mutually exclusive to me. That doesn't mean to me that if you are obese that you are necessarily morbidly unhealthy apart from the obvious risk factors. There are also complications for diving from being too skinny. I have a student in training right now who has about 3% body fat. She is kind of like a humming bird. She needs to eat all the time or she will lose weight. It's hard for her to even gain a few kilograms and if she stops eating for a short time the weight starts to melt off again....
.... naturally .... this makes me feel the searing frustration of injustice. I am seriously jealous of her tall, blonde, skinny, well tattooed, looking amazing in a bikini guts for that. but that's another story. The complication is, of course, that she gets cold very fast.... She said that she would gladly trade with her boyfriend or me (both of us look like fat sad little bacon addicted hobbits compared to her) to which we scoffed in jealous rejection and she insisted that she was serious.... but ok. As you can see I get to know my OW students fairly well as we go
Back to obesity.
Of course you can be obese and strong (look at sumo wrestlers). You can also be obese and in tune with your body. You can be obese and highly agile as many dancing videos on youtube will prove. You can be obese an have a high degree of mental peace (many spiritual leaders have shown us). You can be obese and be a truly amazing human being ... you can be obese and be a utterly earthshaking lover .... but you cannot be obese and fit. Fit may, in fact, be the only thing you cannot be if you are obese.
I also agree with his suggestion that whether or not you gain or lose weight can be reduced to arithmetic. What goes in, what goes out and the different is either positive or negative.
How you choose to approach that (increase calories burned or decrease calories consumed) seems to be less important than keeping track of it. You know the two groups of people who keep the closest track of their caloric intake? Top athletes and people on weight watchers.
As for scuba diving, I've seen divers of every shape and size and I don't see any reason whatsoever that someone who is obese cannot be a diver. There are some complications, like needing extra ballast and finding gear that fits... but in general I don't see being obese as being incompatible with diving. I do see it as being something that generally makes life harder and may prematurely block you from continuing to be a diver as your body ages, but to disqualify someone for being obese alone is not something that I would feel comfortable doing.
R..